FDA Approves Regeneron's Pasatru (Garetosmab) as the First Treatment for Fibrodysplasia Ossificans Progressiva

The U.S. Food and Drug Administration on August 19 approved Regeneron's Pasatru (garetosmab-grts) injection for the treatment of adults with fibrodysplasia ossificans progressiva (FOP). Pasatru is a VelocImmune-derived, fully human monoclonal antibody that blocks Activin A — the ligand that drives the abnormal bone formation that is the central pathology of FOP. The approval, confirmed on the FDA's 「Novel Drug Approvals for 2026」 list and reported by Drugs.com on August 19, is the first targeted therapy ever approved for the disease.

Why FOP Matters Beyond Its Rarity

FOP is an ultra-rare genetic disorder in which soft tissue — muscle, tendons, ligaments — progressively turns into bone. The disease is caused by mutations in the ACVR1 gene, which encodes the Activin A receptor. Episodes of heterotopic ossification (HO) can be triggered by minor trauma, viral illness, or even stretching, locking joints and progressively immobilizing patients. Median life expectancy in historical cohorts has been around 56 years, with most patients wheelchair-bound by their late twenties. The global prevalence is approximately 1 in 1.4 million, with an estimated 8,000 patients worldwide.

What Pasatru Does

Pasatru binds Activin A with high affinity and prevents it from engaging the mutated ACVR1 receptor, blocking the signal that drives heterotopic bone formation. The pivotal study supporting approval, named OPTIMA and run across 11 sites in the U.S., Europe, and Japan, enrolled 95 adults with genetically confirmed FOP. Patients were randomized to receive Pasatru or placebo over a 56-week period, with the primary endpoint being the annualized change in heterotopic ossification volume measured by low-dose CT. Pasatru reduced the annualized new HO volume by 79 percent versus placebo, with a statistically significant benefit emerging as early as 12 weeks. Secondary endpoints, including flare frequency and pain scores, also favored Pasatru.

The Priority Review Voucher

Regeneron was issued a Priority Review Voucher (PRV) with the Pasatru approval, per the FDA's standard mechanism for rare pediatric disease approvals. The PRV can be applied to a future Regeneron application to shorten the FDA review window from ten months to six, and it is also transferable — Regeneron has historically used PRVs strategically, applying some internally and selling others to recoup development cost. The PRV issuance is a signal that the FDA considers FOP to qualify as a rare pediatric disease even though the approval itself is for adults, because the genetic disease manifests from childhood.

What It Means for Patients and the Field

For FOP patients, Pasatru is the first treatment that addresses the underlying biology rather than symptom management. The drug is administered as a subcutaneous injection every two weeks after a loading-dose phase, with a list price of approximately $475,000 per year in the U.S. — consistent with other ultra-rare disease therapies. Regeneron has committed to a comprehensive patient-assistance program that includes co-pay support for commercially insured patients and free drug for uninsured patients who meet income criteria. For the broader heterotopic-ossification field, Pasatru's approval validates Activin A as a therapeutic target and opens the door to additional programs in acquired HO (for example, post-trauma and post-amputation HO).

The Next Pipeline Implications

Regeneron is also running a pediatric extension of OPTIMA and has signaled an sBLA submission for pediatric FOP in 2027. The Activin A pathway is now being explored in additional indications including ankylosing spondylitis and diffuse idiopathic skeletal hyperostosis (DISH), both of which involve abnormal bone formation. The Pasatru approval is the first U.S. approval of an Activin A blocker in any indication and provides a regulatory template for follow-on therapies targeting the pathway.